A general problem in mass spectrometry is the interference caused by isobars, which are atoms or molecules which have very nearly (substantially) the same mass as the atom or molecule being analyzed (the analyte). Such interferences occur in systems which analyze cations (positively charge ions), anions (negatively charged ions) or both.
Accelerator mass spectrometry (AMS) is the term often applied to a collection of techniques, based upon the use of negative ions, a tandem accelerator and mass spectrometry, that makes possible the measurement of isotopic ratios well below 10−12. The methods have been described, for example, U.S. Pat. No. 4,037,100 to K. H. Purser; A. E. Litherland, “Ultrasensitive Mass Spectrometry with Accelerators”. Ann Rev Nucl. and Particle Sci., 30, pages 437-473, (1980). A recent review of AMS techniques, as applied to measurements of the concentration of long-lived isotopes, has been provided by Elmore, D and Phillips, F. M., “Ultra-sensitive Mass Spectrometry”, Science, 296,543 (1987).
A central problem for AMS ultra-high sensitivity detection of rare stable or radioactive atoms is that there is generally an atomic isobar that has substantially the same mass as that of the rare atom to be analyzed (analyte atom). Even though these isobars have a different atomic number, and it might be expected they would be completely eliminated by careful chemistry, the sensitivity of AMS is so great that residual traces of the atomic isobars are often still present in the purified sample. Also, because the mass differences between isobars is extremely small, high transmission arrangements of dispersive electric and magnetic deflection fields seldom have the dispersion needed to provide isobar separation. Thus, the wanted rare ions and the isobaric background ions can pass unattenuated through the whole AMS system and into the final detector.
An example of the use of such an isobar problem is the measurement of the long-lived isotope 36Cl present in underground aquifers. 36Cl is introduced to the biosphere through spallation of 40Ar by cosmic rays and can be used to derive the time a sample of water has been away from the surface. There are two stable isobars of the radioactive 36Cl atoms: 36Ar and 36S. Because it does not form negative ions 36Ar is not a problem. However, 36S is strongly electronegative and provides a troublesome background. Even after careful chemical separation the background count rates from the 36S isobar may be many thousands per second compared to the wanted count rates of 36Cl of a few per second or less.
The procedure presently used for eliminating this background is to accelerate the ions to an energy of at least 30 MeV and to use rate-of-energy-loss methods (dE/dx techniques), range methods, complete electron stripping or gas filled magnets to distinguish individual 36Cl and 36S events. To realize such energies requires the use of expensive nuclear physics accelerators operating at voltages between 6-10 million volts, with the larger voltage preferred when the ratio of 36S/36Cl is high. Such equipment is physically large, very expensive, is found only at major nuclear facilities and requires the services of a large professional staff for operation and maintenance. While isobar separation using these techniques is possible for lighter analyte ions, for the heavier ions, isobaric backgrounds often establish a significant limitation to ultimate detection limits.
A common and well-known use of accelerator mass spectrometry is analysis of small quantities of carbon 14 for carbon-dating purposes. Although requiring only moderate acceleration voltage for isobar removal, typically 500 kV-3 MV, 14C AMS instruments are still large, complicated and expensive. Currently, important, new applications of 14C analysis are being developed using these conventional AMS instruments. In particular, drugs labeled with 14C tracers are dosed into human patients at very low levels of concentration, and are analyzed by AMS to determine their metabolic fate. This technique is called microdosing and is expected to have great impact on the approach to drug discovery and development.
Sputter ion sources are commonly used for AMS. They produce high current ion beams at moderately high energy, typically 20-30 kV, with an ion energy spread of tens of eV. Prior art approaches to isobaric separation for ion beams from near-thermal sources separate isobars from much lower velocity incoming beams. These approaches do not have the high sensitivity of AMS.
Compact microwave sources are also being developed for use with AMS systems as they produce large ion currents efficiently from gas phase sample materials. However, they only produce anions efficiently and so must be followed by a charge change canal to produce the anions required for 14C isobar separation (14N) and injection into the accelerator section of the accelerator mass spectrometer, with a resulting loss of efficiency See S-W Kim, R. J. Schneider, K. F. von Reden, J. M. Hayes and J. S. C. Wills, Test of negative ion beams from a microwave ion source with a charge exchange canal for accelerator mass spectrometry applications, Rev. Scientific Instruments 73 (2002) 846-848 and references therein.
Thus, there exists a need for a device to separate isobaric interferences using higher energy ion sources with few of the size, safety and cost disadvantages associated with the conventional AMS separation techniques. Such a device and method also would improve the sensitivity and usefulness of the mass spectrometer and have wide applicability and higher sensitivity than conventional methods that are commonly available.